https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8901252/

It is difficult to provide a valid estimation of real frequency. There are only a few own observations in the literature and a lot of citations.

We performed a search in our radiologic database, looking for situs inversus as key words in the results. Between 2006 and 2020, 217,646 imaging examinations (ultrasound, CT and plain radiography) were performed at the Department of Transplantation and Surgery, Semmelweis University. Out of them, 21 cases were found, which represents a 1:10,000 frequency. This hospital-based prevalence rate best reflects Adams et al in 1937 (23:232,113), and Lin et al in 2000 (20:201,084) from Massachusetts, as data from own observations.26 This rate is similar as well to the population-based Baltimore-Washington Infant Study.12 SIT is slightly more frequent in males: 1.5:1.27

https://www.healthline.com/health/situs-inversus#symptoms

Because the condition seldom causes symptoms and is so rare, a person may not know they have it. And it may not be discovered until visiting a doctor for a different reason.

https://my.clevelandclinic.org/health/diseases/23486-situs-inversus

You may not develop any symptoms with situs inversus. Although your organs are reversed, they’re often still functional. So you wouldn’t notice any signs or complications.

Of course, trying to estimate how many people don't know about a disease is a difficult task, but the general consensus is the condition is rare and often doesn't produce any symptoms, as such there are definitely many people with the condition that haven't even ever heard of it.